Inkjet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner onto an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the receiver member to yield the desired image. In one process, known as drop-on-demand inkjet printing, individual droplets are ejected as needed onto the recording medium to form the desired image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation using heated actuators. With regard to heated actuators, a heater placed at a convenient location within the nozzle or at the nozzle opening heats ink in the nozzle to form a vapor bubble that causes a drop to be ejected to the recording medium in accordance with image data. With respect to piezoelectric actuators, piezoelectric material is used in conjunction with each nozzle and this material possesses the property such that an electrical field when applied thereto induces mechanical stresses therein causing a drop to be selectively ejected from the nozzle selected for actuation. The image data provides signals to the printhead determining which of the nozzles are to be selected for ejecting an ink drop, such that each nozzle ejects an ink drop at a specific pixel location on a receiver sheet.
In another process, known as continuous inkjet printing, a continuous stream of droplets is discharged from each nozzle and deflected in an image-wise controlled manner onto respective pixel locations on the surface of the recording member, while some droplets are selectively caught and prevented from reaching the recording member. Inkjet printers have found broad applications across markets ranging from the desktop document and pictorial imaging to short run printing and industrial labeling.
A typical inkjet printer produces an image by ejecting small drops of ink from the printhead containing spatial array nozzles, and the ink drops land on a receiver medium (typically paper, coated paper, etc. and referred to generically here as paper or page or media) at selected pixel locations to form round ink dots. Normally, the drops are deposited with their respective dot centers determined by a rectilinear grid, i.e. a raster, with equal spacing in the horizontal and vertical directions. The inkjet printers may have the capability to either produce dots of the same size or of variable size. Inkjet printers with the latter capability are referred to as multitone or gray scale inkjet printers because they can produce multiple density tones at each selected pixel location on the page.
Inkjet printers may also be distinguished as being either pagewidth printers or swath printers. Examples of pagewidth printers are described in U.S. Pat. Nos. 6,364,451 B1 and 6,454,378 B1. As noted in these patents, the term “pagewidth printhead” refers to a printhead having a printing zone that prints one line at a time on a page, the line being parallel either to a longer edge or a shorter edge of the page. The line is printed as a whole as the page moves past the printhead and the printhead is typically stationary, i.e. it does not transverse the page. These printheads are characterized by having a very large number of nozzles. The referenced U.S. patents disclose that should any of the nozzles of one printhead be defective the printer may include a second printhead that is provided so that selected nozzles of the second printhead substitute for defective nozzles of the primary printhead.
A swath printer uses a printhead having a plurality of nozzles disposed in an array in one or more rows, such that the length of the array is somewhat less than the height of the page. The multiple rows can be nozzles for ejecting different ink colors or different droplet sizes. Multiple rows are also used to increase the effective nozzle density for printing by staggering the rows of nozzles along the length of the array. Because the array length is less than the height of a page, printing is done in swaths having a height, which is equal to or less than the array length. A swath is printed as the printhead traverses across a page to be printed in a traversal direction, which is substantially perpendicular to the array length. The printhead traversal direction is also referred to as the fast scan direction. After the swath is completed, the paper is advanced along a paper movement axis, which is perpendicular to the printhead traversal direction. The paper movement axis is also called the slow scan direction. The distance of paper advance is set to be less than or equal to the swath height in order to allow every pixel location on the page to be printed in successive swaths. For fastest printing throughput, all pixels to be printed in the region traversed by the printhead are printed during a single pass, and the page advance is set to the swath height. However, in many applications it is found that print quality is improved if a subset of pixels is printed in each pass, and multiple passes are used to print each region. In multi-pass printing, the page advance distance is set to be less than the swath height.
There are many techniques present in the prior art that describe methods of controlling the printer including “print masking.” The term “print masking” generally refers to printing subsets of the image pixels in multiple passes of the printhead relative to a receiver medium. The print mask indicates which pixels have permission to be printed during a given pass of the printhead.
When printing on a cut-sheet inkjet printer, the paper is held by (at least) two sets of rollers. The first set is made up of a long main roller below the paper and one or more rollers above. The upper rollers are tensioned against the lower roller and are free turning. The lower roller is driven to advance the paper. The second set of rollers has a long main roller below the paper and one or more star wheels above the paper. The star wheels are tensioned against the lower roller and are free turning. The second upper set are star shaped to minimize contact with the freshly printed paper surface and to avoid smearing the ink.
As the paper is fed through the printer, it starts out held by only the first roller set. In this portion of the printing process, the paper may curl up or down, changing the head/paper spacing which changes dot alignment. Part way into the print, the paper will start being held by the star wheel rollers also. This middle area of the print is the most stable for paper advance and head/paper spacing since the paper is held by both sets of rollers. Then, at the end of the print, the paper comes out of the first roller and is only held by the star wheel rollers. At this point, paper curl could change the head/paper spacing. Also, the paper advance distances may not be as accurate when the paper is only held by the star wheel rollers.
One method of solving this in the past was by changing to different print modes at the leading and/or trailing edges of the print. A print mode is defined as the combination of the print mask size, print mask data, and page advance distance. One problem with this approach is that it requires the paper to be advanced in a short/long/short/etc sequence when weaving the end of one print mode into the next print mode. Changing paper advance distances up and down can introduce feed errors. Also, the number of nozzles printing from one pass to the next varies significantly and down by a large percentage of the printhead. This can cause electrical current and thermal effects in the printhead.